1. Field of the Invention
This invention relates to a composite substrate material comprising at least two substrates which are bonded to each other. This invention also relates to a process for producing the composite substrate material.
2. Description of the Related Art
Composite materials have heretofore been formed by bonding optical crystals, such as laser crystals, optical wavelength converting crystals, and quartz glass mirrors, to each other or by bonding optical substrates to each other. As techniques for the bonding, techniques, wherein the optical crystals or the optical substrates are adhered to each other by optical adhesive agents, or techniques, wherein the optical crystals or the optical substrates are fusion bonded under heat, have heretofore been used widely.
However, the techniques, wherein the optical crystals or the optical substrates are adhered to each other by optical adhesive agents, have the problems in that optical scattering and reflection loss are caused to occur and long-term reliability of the adhered areas is low. Particularly, as for optical members to be located within laser resonators, the problems described above arise markedly.
The techniques, wherein the optical crystals or the optical substrates are fusion bonded under heat, have the problems in that the techniques are applicable only to limited combinations of materials and can be utilized only in limited applications.
Therefore, recently, techniques for bonding different kinds of materials to each other, which techniques are referred to as wafer bonding techniques, have attracted particular attention. With the wafer bonding techniques, as described in, for example, Japanese Unexamined Patent Publication No. 6(1994)-90061, wafers of single crystals or polycrystals are subjected to mirror finish, the mirror surfaces thus obtained are washed to remove dust and organic substances and are set in a hydrophilic state, the mirror surfaces are then brought into contact with each other in a clean atmosphere, and the wafers are heated in this state.
With the wafer bonding techniques described above, a composite substrate material having a high bond strength can be formed. However, the wafer bonding techniques have the problems in that wet processing must be performed for cleaning the substrates and therefore production steps cannot be kept simple.